1. Field of the Invention
The present invention relates to a wavelength conversion device composed of single-crystal lithium tetraborate (Li2B4O7) and a laser apparatus provided with the wavelength conversion device, and also to a method of converting a wavelength using the same.
2. Description of the Related Art
Short-wavelength solid-state lasers operating in the ultraviolet and visible regions, which are provided with infrared solid-state lasers, such as YAG lasers, and wavelength conversion devices for reducing the wavelength of incident light by half have advantages over conventional argon lasers and excimer lasers because they are very safe, easy to maintain, inexpensive, and small and compact. Accordingly, researches have been intensively conducted on short-wavelength solid-state lasers.
In particular, a method of generating continuous wave (CW) ultraviolet light by second harmonic generation (SHG) of an argon laser is receiving attention. In this method, continuous light that is not obtainable by an excimer laser can be generated, and a laser beam in a wavelength band of 240 nm, which is indispensable for fiber gratings, can be generated. Thus, such generation will be an important light source for laser material processing in future.
Currently, only BBO (BaB2O4) is used as a wavelength conversion crystal for producing the second harmonic of the argon laser. The crystal has a large nonlinear coefficient, which is advantageous.
However, since the wavelength conversion device composed of the BBO crystal is a critical phase matching (CPM) type device, the angular bandwidth is small, the walk-off angle is large, and thin crystals must be used. Thus, it is not possible to increase the conversion efficiency. The angular bandwidth refers to an angle (unit: mradxc2x7cm) obtained when an output of a laser beam, in which the wavelength is converted when a phase matching angle (an angle between the laser beam and the c-axis when the wavelength conversion device produces a second harmonic) is rotated by a microscopic angle, corresponds to half of the output of the second harmonic at the exact phase matching angle. Furthermore, the BBO crystal itself generates heat by absorbing the second harmonic, and thus the phase matching conditions are changed, resulting in output variation in the second harmonic. The crystal also has a short life. Therefore, although the generation of continuous wave ultraviolet light by the second harmonic of the argon laser is very effective, the industrial use thereof has not yet been implemented.
It is an object of the present invention to provide a wavelength conversion device composed of single-crystal lithium tetraborate which is suitable for wavelength conversion by non-critical phase matching (NCPM) and which has a large angular bandwidth and excellent durability to laser light.
It is another object of the present invention to provide a laser apparatus which stably generates a continuous wave laser beam having a wavelength of 244 nm or 248.25 nm, using a wavelength conversion device composed of single-crystal lithium tetraborate in which wavelength conversion is performed by non-critical phase matching.
In one aspect of the invention, as shown in FIG. 1, a wavelength conversion device 10 is composed of single-crystal lithium tetraborate which is cut so that the direction of propagation of light is set in the direction satisfying the relationship xcex8m=90xc2x0xc2x12xc2x0, where xcex8m is an angle between the direction of propagation of light and the c-axis.
In accordance with the present invention, with respect to the single-crystal lithium tetraborate cut in such a manner, by orienting an incident laser beam in the direction of propagation of light at a predetermined temperature, beam walk-off does not occur, and the wavelength conversion device 10 which is suitable for wavelength conversion by non-critical phase matching is obtained.
In another aspect of the invention, as shown in FIG. 2, in a laser apparatus in which the wavelength is converted by passing an incident laser beam through a wavelength conversion device 10 composed of single-crystal lithium tetraborate, the relationship between the orientation of the crystal axis and the direction of incidence of the laser beam is determined so as to prevent beam walk-off from occurring in the laser beam propagating through the conversion device 10.
Preferably, as shown in FIG. 2, in the wavelength conversion device 10 of the laser apparatus, the single-crystal lithium tetraborate is cut so that the direction of propagation is set in the direction satisfying the relationship xcex8m=90xc2x0xc2x12xc2x0, where xcex8m is an angle between the direction of propagation and the c-axis.
In accordance with the present invention, by orienting an incident laser beam in the direction of propagation of light at a predetermined temperature, beam walk-off does not occur in the wavelength conversion device 10, and the laser apparatus using non-critical phase matching is obtained.
Preferably, the laser apparatus includes a temperature regulator 13 for maintaining the wavelength conversion device 10 composed of single-crystal lithium tetraborate within a predetermined temperature range.
Accordingly, the wavelength conversion device 10 is maintained at a predetermined temperature by the temperature regulator 13.
In another aspect of the invention, a method of converting a wavelength by passing an incident laser beam through a wavelength conversion device 10 composed of single-crystal lithium tetraborate includes the steps of determining the relationship between the orientation of the crystal axis and the direction of incidence of the laser beam so that beam walk-off does not occur in the laser beam propagating through the wavelength conversion device 10, maintaining the wavelength conversion device 10 in a temperature range of 13.5xc2x110xc2x0 C., preferably 13.5xc2x12xc2x0 C. and passing a laser beam having a wavelength of 488 nm through the wavelength conversion device 10 so that the wavelength is converted into 244 nm.
Accordingly, a continuous wave laser beam having a wavelength of 244 nm can be generated stably.
In another aspect of the invention, a method of converting a wavelength by passing an incident laser beam through a wavelength conversion device 10 composed of single-crystal lithium tetraborate includes the steps of determining the relationship between the orientation of the crystal axis and the direction of incidence of the laser beam so that beam walk-off does not occur in the laser beam propagating through the wavelength conversion device 10, maintaining the wavelength conversion device 10 at a temperature of 300xc2x0 C. to 500xc2x0 C., and passing a laser beam having a wavelength of 496.5 nm through the wavelength conversion device 10 so that the wavelength is converted into 248.25 nm.
Accordingly, a continuous wave laser beam having a wavelength of 248.25 nm can be generated stably.
Preferably, in either one of the methods of converting a wavelength described above, as shown in FIG. 1, the single-crystal lithium tetraborate is cut so that the direction of propagation is set in the direction satisfying the relationship xcex8m=90xc2x0xc2x12xc2x0, where xcex8m is an angle between the direction of propagation and the c-axis.
Accordingly, a continuous wave laser beam having a wavelength of 244 nm or 248.25 nm can be generated stably by non-critical phase matching.